Considerable work has been undertaken in connection with providing a connection or coupling between a removable partial dental prosthesis and the abutment tooth or teeth defining an edentulous space in which the prosthesis is to be retained. It is generally recognized in the literature that if the stress on a partial denture is passed or transmitted directly to the abutment tooth, the stress will break down the periodontal fibers supporting the abutment tooth, with resultant eventual loss of the tooth. In order to avoid this problem, various means for connecting partial dental prosthesis to an abutment tooth have been developed, including the devices set forth in U.S. Pat. Nos. 4,293,303, 3,412,468 and 1,545,734. Additional stress relieving connections are disclosed in Italian Patent Nos. 541,013, 502,531 and 489,861.
In our prior above-identified patent application, a removable partial dental prosthesis is disclosed in which the connection between the prosthesis and the abutment tooth is provided by a cantilevered arm structure. The connection includes a resilient element which enables limited universal linear and articulated movement of the prosthesis with respect to the abutment tooth so as to reduce the stress passed through the connection to the abutment tooth and the periodontal fibers. In this prior dental prosthesis, the prosthesis is coupled to the abutment tooth by a standard dental clasp. The function of the dental clasp is to retain the prosthesis in the edentulous space by resilient lateral engagement with the sides of the abutment tooth to prevent the denture from slipping vertically over the abutment tooth under the action of gravity and/or sticky food.
A clasp for a partial dental prosthesis typically includes a retention arm which is cantilevered from the edentulous side of the abutment tooth and passes around and encircles part of the tooth. The retention arm is resiliently flexible so as to grip the tooth and retain the prosthesis against removal. The retention arm, however, must also be flexible enough to enable the wearer to pull the clasp over the abutment tooth for removal of the prosthesis.
A prosthodontist conventionally employs a device called a parallelometer or surveyor to determine and trace the parallel line around the greatest circumference of the abutment tooth in a horizontal plane. This line is called the "survey line," and the area below the survey line is the undercut area, with the area above being the bracing or stabilizing area. Typically, the retention arm of a standard clasp encircles the abutment tooth with its upper edge above or extending along the survey line and the lower edge and free end of the arm below the survey line.
Retention arms are usually generally semicircular in cross-section with the flat side of the semicircle engaging the tooth surface, and are cantilevered from a vertically extending post portion of the clasp which is positioned next to the edentulous space. Standard clasps also typically include a rest or horizontally extending protrusion from the top of the vertical post which engages the top of the tooth and limits vertical displacement toward the gum line. Most clasps also include a second or reciprocal arm or band which is cantilevered from the post on an opposite side of the abutment tooth from the retention arm.
In operation, a conventional clasp flexes laterally with respect to the surface of the abutment tooth as it passes over the survey line and resiliently springs back against the tooth once in the undercut area to frictionally retain the denture in the edentulous space.
As above set forth, the improved dental prosthesis of our prior application involves the positioning of a universally displaceable connection between the standard dental clasp and the prosthesis. The present invention relates to an improvement in the clasp itself and thereby to an improvement in the connection between the prosthesis and the clasp.
One of the problems which is prevalent in a conventional dental clasps for a removable partial prosthesis is that the lateral resiliency inherent in the cantilevered retention arm is relatively limited. In the clasp structure, the distance to the free end of the retention arm from the vertical post portion of the clasp is very small. Accordingly, it is not possible to build in very much resiliency, and attempts to enhance this resiliency have been largely directed toward flattening the retention arm cross-section so as to make it somewhat more flexible. An additional advantage of such flattening is that more of the abutment tooth will be engaged by the band, which enhances the frictional retentive effect of the clasp.
A disadvantage of such clasp constructions, however, is that because of the lack of resiliency, the band must be placed fairly close to the survey line or relatively high on the abutment tooth. This means that the retention arm band will often be quite visible and cosmetically objectionable to the wearer. The result is often that the user of the dental prosthesis does not wear the same as often as would otherwise be desirable.
Another disadvantage of prior flexible clasp constructions for partial dental prosthesis has been that it is difficult to generate enough retention force in the clasp to withstand the forces tending to pull the clasp off of the abutment tooth because the limited flexibility does not permit engagement of the abutment tooth to any substantial degree in the undercut area. Accordingly, mastication of sticky foods can be particularly troublesome with respect to the retention of removal of partial prosthesis in the upper jaw.